1. Field of the Invention
The present invention generally relates to reel-deployed printed circuit boards for chip-on-board (COB) packages and a method for manufacturing IC packages using such circuit boards.
2. Description of the Related Art
In many applications, such as telephone cards, credit cards and identification cards, integrated circuit (IC) cards can advantageously replace conventional magnetic strip cards. The IC card typically includes a "chip-on-board" (COB) package in which a semiconductor integrated circuit chip is connected to a printed circuit board and is encapsulated with an encapsulant. COB packages are manufactured in several different ways, depending on the type of printed circuit board and the encapsulating methods used. Among these, a COB package using a reel-deployed printed circuit board is preferred due to lower production costs and higher assembly productivity.
FIG. 1 is an exploded perspective view of an IC card 10 which has a card body portion 11 with a COB package 20 assembled into it. FIG. 2 is a cross-sectional view of the IC card 10 shown in FIG. 1. FIGS. 3 and 4 are top and bottom views, respectively, of a COB package 20 of the type shown in FIGS. 1 and 2. As shown in FIG. 1, the COB package 20 is inserted into a cavity in the body 11 of the card 10. Thus, the card body 11 includes an adhesion area 12 to which the COB package 20 is attached, and a cavity 13 for receiving an encapsulated portion 24 of the COB package 20.
Referring to FIG. 2, in manufacturing the COB package 20, a semiconductor chip 21 is attached to a top surface of a printed circuit board 30 by an adhesive 22, and bonding wires 23 electrically connect the semiconductor chip 21 to bonding pads 31 formed on the top surface of the printed circuit board 30. The semiconductor chip 21, bonding pads 31, and bonding wires 23 are all then encapsulated with an encapsulant to seal and protect them from the environment. As shown in FIG. 4, the other side, that is, a bottom surface of the printed circuit board 30, typically has a plurality of contacts 33 which are exposed on the card when the COB package 20 is assembled into the card body 11 in the orientation shown. The contacts 33 are electrically connected to the bonding pads 31 on the top surface by plated-through holes 32, sometimes called "via holes." Non-conductive gaps 34 on the bottom surface electrically isolate the contacts 33 from each other. As shown in FIGS. 3 and 5, a dam 35 prevents runoff of the liquid encapsulant and gives shape to the encapsulated portion 24 of the package.
FIG. 5 shows a conventional reel-deployed printed circuit board 40 which is used to manufacture a plurality of the COB packages 20 simultaneously in successive manufacturing steps, as depicted from left to right in the figure. In FIG. 5, reference numeral 37 indicates the outline of an individual COB package 20.
To prepare the reel-deployed printed circuit board 40 for the encapsulation step, a dam 35 is formed on the top surface of the board. Among the various encapsulation methods available, a method that dispenses a liquid encapsulant onto the semiconductor chip and the wire bonding area and then cures the encapsulant in place, is preferred for COB packages that use a reel-deployed printed circuit board 40. In such a process, the dam 35 prevents runoff of the liquid encapsulant and acts as a mold to shape the encapsulated portion outlined by the dam. The printed circuit board 40 further includes sprocket holes 41, position holes 42, assembly-reject holes 43a and test-reject holes 43b, the functions of which are described below.
An "in-line" system such as that described above can theoretically manufacture COB package 20 without any interruptions between consecutive assembly steps. However, as a practical matter, an unbalanced production capability among the steps can make operation of the in-line system difficult and inefficient. Further, an interruption of the line at any one point can halt the entire assembly line, and thereby reduce the productivity of the entire manufacturing process. In view of this, an alternative manufacturing method, one that performs each manufacturing process, or step, independently of the others, and which uses a reel hub 50, such as the one shown in FIG. 6, for conveniently storing the reel-deployed printed circuit board 40 between manufacturing steps, and for transferring it from step to step, becomes more preferable.
In this alternative method, a reel-deployed printed circuit board 40 is initially provided in the form of a long strip wound onto a spool, as shown in FIG. 6. At each independent step, or stage, in the manufacturing process, the printed circuit board 40 is wound off of the reel hub 50, a process step is performed on it, and the board 40 is then wound onto another reel hub 50 for interim storage and conveyance to the next-succeeding manufacturing step.
The reel hub 50 thus may be seen as an almost indispensable tool for the practical mass-assembly of COB packages of the type that use a reel-deployed printed circuit board 40. Without it, the assembly could, at best, be carried out using only a short, strip-type printed circuit board. COB assembly methods using reel-deployed printed circuit boards 40 usually result in a much higher productivity than assembly methods using a strip-type printed circuit board. For example, a reel hub having a diameter of 330 mm is typically capable of holding about 30,000 COB packages 20 simultaneously.
In FIG. 6, reference numeral 40a indicates the top surface of a reel-deployed printed circuit board 40 on which the encapsulated portion 24 is formed, and reference numeral 40b indicates the bottom surface of the circuit board 40 on which the contacts 33 are formed. Those skilled in the art will recognize that it may be desirable to temporarily insert an optional "interposer" (not shown) over the circuit board 40 after the die-is attached and wire-bonded but before it is encapsulated, to protect the fragile bonding wires 23 (see FIG. 2) until the encapsulant is in place. The interposer can then be removed after the encapsulation is complete.
Although the use of reel-deployed printed circuit boards 40 can enhance the productivity of COB packages 20, their use can also result in an adverse effect. In particular, when the reel-deployed printed circuit board 40 is wound off of the reel hub 50, the board 40 may not be desirably flat, but rather, curved to a certain extent. That is, the circuit board 40 may take an arcuate "set" while it is wound on the circular hub of the reel that remains in the board when it is removed from the reel.
FIG. 7 illustrates the undesirable curvature that a conventional reel-deployed printed circuit board 40 can assume after it is removed from a storage reel. Typically, the encapsulation and curing steps produce a slight warpage of the COB package 20 anyway, because of a mismatch of thermal expansion coefficients between the encapsulant and the semiconductor chip 21. Thus, a warped printed circuit board 40, such as that shown in FIG. 7, can then exacerbate the warpage of the COB package 20. This warpage can cause an attachment failure between the card body 11 and the COB package 20, as shown in FIG. 8. Further, any protrusion of the COB package 20 from the surface of IC card 10 caused by this warpage of the COB package body will promote premature wear of the contacts 33, and thereby decrease the usable life and reliability of the IC card 10.